It is well known in the art that in a CDMA system the uplink (mobile station to base station) is interference limited. Further, the total interference received at the base station should not exceed a certain threshold, otherwise the system will become overloaded. FIG. 1 shows an example of sampled system load variations with time. It is desirable that the system load should always be maintained below the threshold and, to most efficiently make use of the allocated bandwidth, whenever there is system margin available it should be immediately utilized.
As is also well known, it is typically the case that the data traffic between a terminal and the system is asymmetric, i.e., the amount of uplink data traffic is typically much less than the amount of downlink data traffic. For example, a terminal user may be required to input but a few keystrokes in order to download a large text and/or graphics file, such as a web page. As such, the typically low data volume service in the uplink direction requires only a low to medium data rate. Examples of such a low volume data service include HTTP requests and Internet traffic. These and similar services could be readily accommodated by the available system margin shown in FIG. 1.
If a reverse data bearing channel is employed for the uplink data service, a certain amount of time is required to set up and release the channel, as well as for the power control technique to stabilize. However, the low volume of data traffic does not justify the resources and time required for setting up the reverse data bearing channel. Also, if the available system margin is small (e.g., time A in FIG. 1), the access to a data bearing channel may be refused by the wireless network. Moreover, it is possible that by the time the reverse data bearing channel has been set up the system margin may already have been reduced to an unacceptable level (e.g., channel set up is requested at time B, and is completed at time C in FIG. 1).
As such, it can be appreciated that the reverse data bearing channel may not be an optimum resource for use in servicing uplink data traffic.
The R-EACH is conventionally used in the IS-2000 physical layer for various functions, including the frequent transmission of short packets. In the prior art both the basic and reserved access modes can be used for sending short uplink messages. However, their performance is inferior.